gexp: 'gexp->script' and 'gexp->file' have a new #:module-path parameter.

* guix/gexp.scm (load-path-expression): Add 'path' optional parameter.
(gexp->script): Add #:module-path and honor it.
(gexp->file): Likewise.
* tests/gexp.scm ("gexp->script #:module-path"): New test.
* doc/guix.texi (G-Expressions): Update accordingly.